The versatile skulls of herbivorous fishes: functional morphology of pacu jaws and teeth

Abstract

The Serrasalmidae are a family of freshwater Neotropical fishes that includes carnivorous piranhas as well as their herbivorous relatives, the pacus. Pacu diets consist of leaves, stems, fruits, seeds, and algae, as well as insects, benthic invertebrates, plankton, and smaller fish if available. These diverse prey materials appear concomitant with diverse jaws and dentitions, suggesting that some species are ecomorphologically specialized for feeding on certain prey. We investigated how the pattern and tempo of feeding morphological specialization in pacus reflects the ecological diversity of their food resources. Pacu species were first categorized as either algivores, frugivores, folivores, phytophages, planktivores, or omnivores based on a meta-analysis of published gut content data. We then used CT scanning and linear morphometrics to describe the primary morphological axes of jaw and dental variation, and any correlates these phenotypes may have with each species’ primary prey. We found significant differences in the occlusional offset, opening mechanical advantage, jaw width, and sagittal crest height between the different diet guilds. The phytophages tended to have lower occlusional offsets, and showed higher resemblance to piranhas than other pacus with scissor-like occlusion. Additionally, we found significant differences in the rates of morphological disparity through time among the different diet groups. Folivores had morphological rates of evolution over 40 times faster than that of planktivores, suggesting different selective regimes acting on each group in association with diet. These findings indicate that because different species of pacu have different diets, we cannot expect their dentitions to look and function in the same ways.